Casing Exit Mill Assemblies with Replaceable Blade Sleeve

ABSTRACT

A mill assembly having a mill shaft body, removable bearing and removable blade sleeve. The shaft body presents a mounting portion having a plurality of arcuate curved contact faces for transmission of rotational forces.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the design and construction ofdownhole milling tools used to perform casing exits and other metalcutting operations.

2. Description of the Related Art

Conventional large size casing exit mills have a cylindrical body with alarger diameter section that transitions to a smaller diameter pipe onboth sides at a taper angle. Blades are welded upon the taper and anenlarged section. The blades are typically brazed with crushed tungstencarbide particles or a tungsten carbide insert of a particular shapesuitable for cutting metal. Blades are welded onto the body, typicallywith a ⅜″ fillet weld. The brazing of carbide onto the blades is carriedout usually at temperatures that are from about 1650° F. to 1700° F. Ifnot performed in a uniform manner, that high temperature heating processresults in bowing of the body and softening of the steel adjacent to theblades. Though mills are often heat treated after welding and brazing,consistency in the mechanical properties on the surface of the mill bodyis doubtful. Every heat cycle on the mill body and associated componentschanges the mechanical strength of the surface fibers. During a casingexit operation, the mill is subjected to cyclic bending loads andintermittent torsional loads. These loads induce cyclic bending stressand torsional stress in the surface fibers. The superimposition of theaxial component of torsional stress and bending stress causes thesurface fibers to fail resulting in cracks on the body. Hence, it isnecessary for the surface fibers to maintain their mechanical strengthduring each casing exit operation. The higher the number of heat cycles,the higher the tendency for surface fibers to become softer, thus makingthe body susceptible to cracking under low bending stress. Thisreduction in the strength of surface fibers reduces the fatigue life ofthe body.

SUMMARY OF THE INVENTION

The invention provides improved designs for the construction of downholemills. In other aspects, the invention provides methods of assembling amill. An exemplary casing exit mill assembly is described in which acentral shaft body is provided with a mounting portion that is shaped toprevent rotation of a blade sleeve mounted thereupon.

In described embodiments, the mounting portion presents a plurality ofarcuately curved contact surfaces. Adjacent contact surfaces on themounting portion adjoin each other at angled corners. There are at leastthree curved contact surfaces. According to preferred embodiments, thereare between three and twelve contact surfaces. The cross-sectional shapeof each of the contact surfaces is defined as an arcuate segment from acircle having a radius which is greater than the radius of the contactsurface upon the shaft body mounting portion.

A bearing and a blade sleeve surround the mounting portion. The bearingcould be a separate component from the shaft and blade sleeve. In otherembodiments, the bearing is a coating formed upon either the mountingportion of the shaft body or upon the inner surface of the centralopening of the blade sleeve. The blade sleeve is provided with a centralopening that is shaped and sized to be complementary to the mountingportion of the shaft body. Preferably, the components are securedtogether using a press fit or interference fit. The modular constructionof the mill assembly permits the blade sleeve to be easily replaced whenworn or damaged. Alternatively, the blade sleeve could be replaced by ablade sleeve having a different diameter or design of cuttingstructures.

The inventor has found that the use of a mounting portion havingarcuately curved contact surfaces and a blade sleeve having acomplementarily shaped engagement surface is advantageous. Torque forcescan be effectively transmitted between the components while minimizingthe stress that might result from other interfaces.

In certain embodiments, the shaft body of the mill assembly includes aradially enlarged portion which is located proximate the blade sleeve. Aremovable protection blade sleeve radially surrounds the radiallyenlarged portion. Like the blade sleeve, the protection blade sleeve ispreferably press fit and features a tapered interface with the radiallyenlarged portion.

BRIEF DESCRIPTION OF THE DRAWINGS

For a thorough understanding of the present invention, reference is madeto the following detailed description of the preferred embodiments,taken in conjunction with the accompanying drawings, wherein likereference numerals designate like or similar elements throughout theseveral figures of the drawings and wherein:

FIG. 1 is a side, cross-sectional view of an exemplary casing exit millassembly constructed in accordance with the present invention.

FIG. 2 is an enlarged side, cross-sectional view of portions of the millof FIG. 1.

FIG. 3 is an axial cross-section of the mill assembly taken along lines3-3 in FIG. 2.

FIG. 4 is an axial cross-section of the mill assembly of FIGS. 1-3, nowbeing subjected to torsional force.

FIG. 5 is a schematic view of the mounting portion of the mill assemblyshown in FIGS. 1-4 in comparison to a larger circle.

FIG. 6 is a side view of the shaft body of an alternative mill assemblywherein the mounting portion of the shaft body has six contact faces.

FIG. 7 is a side view of an alternative mill assembly which incorporatesthe shaft body shown in FIG. 6.

FIG. 8 is an axial cross-section taken along lines 8-8 in FIG. 6.

FIG. 9 is an axial cross-section taken along lines 9-9 in FIG. 7.

FIG. 10 is a further axial cross-section of a mounting portion with sixcontact faces illustrating exemplary geometric features of the mountingportion.

FIG. 11 is a side, cross-sectional view of another enlarged portion ofthe shaft body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1-2 depicts an exemplary casing exit mill assembly 10 constructedin accordance with the present invention. The mill assembly 10 includesa central shaft body 12 which defines an axial fluid passage 14 alongits length. Threaded connections 16 are provided at each axial end ofthe shaft body 12 to permit the mill assembly to be incorporated into adownhole work string. The shaft body 12 is preferably cylindrical inshape along its length except where otherwise indicated. A mountingportion 18 of the shaft body 12 has a shaped outer radial surface uponwhich a bearing 20 and a blade sleeve 22 are mounted. The outer radialsurface of the mounting portion 18 is shaped to preclude rotation of thebearing 20 and blade sleeve 22 with respect to the shaft body 12. Withfurther reference to FIGS. 3-4, it can be seen that the mounting portion18 presents three arcuately curved convex contact faces 24 which adjoinone another at corners 26. The mounting portion 18 is designed to ensurethat the blade sleeve 22 and bearing 20 do not rotate upon the shaftbody 12. It should be appreciated with reference to FIGS. 3 and 4 thatthe mounting portion 18 is generally nearly round but still providescorners 26 that will prevent rotation of the surrounding bearing 20 andblade sleeve 22 upon the shaft body 12.

It is noted that, in axial cross-section (i.e., FIGS. 3-4) the contactfaces 24 of the mounting portion 18 preferably each form an arcuatesegment of a circle that has a radius greater than the radius 25 of themounting portion 18 at its widest point. FIG. 5 illustrates a largercircle 27 of which a contact face 24 forms an arcuate segment. Thelarger circle 27 has a radius 29 that exceeds the radius 25 of themounting portion 18 at its widest point.

In certain preferred embodiment, the radius 29 is at least twice aslarge as the radius 25.

Referring once again to FIG. 1, it is noted that the shaft body 12 alsoincludes a radially enlarged portion 31 which is located proximate, buta spaced distance from, the mounting portion 18. A protection bladesleeve 33 radially surrounds the radially enlarged portion 31 andpresents a hardened protruding portion for engaging a surroundingtubular during casing window cutting. The radially enlarged portion 31and the protection blade sleeve 33 are preferably press fit.

Preferably, the mounting portion 18 is axially tapered to allow for easeof assembly. As illustrated in FIG. 2, the mounting portion 18 istapered at an angle “r”. In currently preferred embodiments, the angleof taper “r” is about 2 degrees, as measured from the central axis ofthe shaft body 12. This taper facilitates a press fit securing of thebearing 20 onto the mounting portion 18. As best seen in FIGS. 3-4, theblade sleeve 22 presents a plurality of cutting blades 28 which projectradially outwardly from the sleeve 22. The cutting blades 28 are formedof or brazed with carbide cutters or other hardened cutting structures.

The bearing 20 is preferably formed of a material that is softer thanthe material making up the shaft body 12 and the blade sleeve 22. Inpreferred embodiments, the bearing 20 is formed of copper, manganese orbronze, or alloys which include these materials. In alternativeembodiments, the bearing 20 is formed of a viscoelastic material whichprovides an effective damper for torsional vibrations and shocks. Thebearing 20 may be in the form of a separate component that is disposedbetween the shaft body 12 and the blade sleeve 22. Alternatively, thebearing 20 may be in the form of a coating that is applied to either orboth of the shaft body 12 and/or the blade sleeve 22.

FIGS. 3 and 4 help illustrate the resistance of the blade sleeve 22 torotation in response to torsional loading. FIG. 3 illustrates the millassembly 10 having no torsional loading applied. In FIG. 4, a torsionalforce is being applied to the blade sleeve 22, as indicated by arrow 30.Bearing 20, if made of a viscoelastic material, deforms slightly toaccommodate the loading. Absorption of torque spikes by the bearing 20will help damp torsional impact loads on the blades 28. The absorptionof torque spikes will essentially result in less wear upon the blades 28and increase the life of the mill assembly 10.

The mill assembly 10 can be constructed by first sliding the bearing 20onto the shaft body 12 and over the mounting portion 18. Thereafter, theblade sleeve 22 is slid onto the shaft body 12 to overlie the bearing 20so that the bearing 20 is located radially between the shaft body 12 andthe blade sleeve 22. A press fit, or interference fit, affixes the threecomponents together.

It is noted that the modular construction of the mill assembly 10permits users to easily replace a worn blade sleeve 22 or to replace theblade sleeve 22 with a blade sleeve having a larger or smaller outerdiameter or having a different type or design of cutting blades orstructures.

In operation, the mill assembly 10 is constructed as described above andis then incorporated into a work string. Thereafter, the work string isdisposed into a wellbore. The mill assembly 10 is run in to a desiredlocation within the wellbore and rotated, in a manner known in the art,so that the blade sleeve 22 of the mill assembly 10 mills or cuts awaydesired material.

In order to remove the blade sleeve 22 from the mill shaft body 12, aswell as the bearing 20, axial force is applied to the blade sleeve 22proximate the larger end of the taper of the mounting portion 18. Arrow32 in FIG. 2 illustrates the application of such force. The forceapplied must be sufficient to overcome the frictional force of the pressfit, thereby causing the blade sleeve 22 and perhaps the bearing 20 tobe unseated from the mill shaft body 12. Thereafter, the blade sleeve 22and/or the bearing 20 could be replaced as necessary.

FIGS. 6-10 illustrate features of an alternative embodiment for a millassembly 50 constructed in accordance with the present invention. Exceptwhere otherwise described, the mill assembly 50 is constructed andoperates in the same manner as the mill assembly 10 describedpreviously. The shaft body 12′ of the mill assembly 50 includes atapered mounting portion 18′. The mounting portion 18′ has six contactfaces 24′ rather than three. The contact faces 24′ adjoin each other atcorners 26′. As FIG. 9 depicts, a bearing 20′ may be disposed betweenthe mounting portion 18′ and the blade sleeve 22′.

FIGS. 8 and 10 help to illustrate geometric features associated with thecontact faces 24′ and the mounting portion 18′ of the mill assembly 50.A true circle 52 is depicted in FIGS. 8 and 10 superimposed around theouter perimeter of the mounting portion 18′. FIG. 10 shows that a gap 54is defined between the contact faces 24′ and the true circle 52. The gap54 results from the fact that the central portions 55 of the contactfaces 24′ are defined as arcuate segments of a circle having a radius 56which is larger than the radius 58 of the mounting portion 18′ at itswidest point. In other words, the contact faces 24′ have a larger radiusof curvature than circle 52. FIG. 10 also illustrates an exemplaryhexagonal outer surface 60 superimposed on the mounting portion 18′ anddrawn with its vertices at the corners 26′. It can be seen that thecontact faces 24′ extend radially outwardly beyond the sides 62 of thehexagonal outer surface 60.

In preferred embodiments, the contact faces 24′ are further shaped topresent contoured transition surface portions 64 that are locatedproximate the corners 26′ of the mounting portion 18′. The transitionsurface portions 64 have a radius of curvature 66 smaller than eitherthe true circle 52 or the central portions 55 of the contact faces 24′.

It is further noted that the mounting portion 18′ is preferably axiallytapered, as illustrated by FIG. 6, in the same manner as described formounting portion 18. The tapering will aid in assembly and repair of themill assembly 50.

The inventor has determined that the use of arcuately curved contactfaces in a mounting portion along with angled corners which adjoin thecontact faces is an advantageous design for transmission of torqueforces between the shaft body and the surrounding blade sleeve. Stressconcentrations which are associated with other designs are avoided. Inparticular, shaft body/blade sleeve interfaces constructed in accordancewith the present invention provide increased contact area between thecontact faces 24, 24′ and the surrounding blade sleeve 22, 22′, therebyincreasing the ability of torque forces to be transmitted between thecomponents and supplementing force transmission between the corners 26,26′ and the surrounding blade sleeve 22, 22′. According to certainpreferred embodiments, contact faces, such as contact faces 24′ haveouter radial portions with different radii of curvature. For example,the central portion 55 of each contact face 24′ has a radius ofcurvature 56 that is greater than the radius of curvature 58 of themounting portion 18′, as measured from its widest point. Lateralportions 64 of the contact face 24′, however, have a radius of curvature66 that is smaller than the radius of curvature 58 for the mountingportion 18′ and the radius of curvature 56 of the central portion 55 ofthe contact face 24′.

In particular embodiments, the mounting portions 18, 18′ have at leastthree contact faces or the type described previously. In preferredembodiments, there are from three to twelve contact faces. Inparticularly preferred embodiments, there are from three to six contactfaces.

FIG. 11 illustrates the radially enlarged portion 31 and the protectionblade sleeve 33 in greater detail. The radially enlarged portion 31preferably presents an outer radial surface that is shaped with contactfaces and corners similar to those described previously with respect tothe mounting portions 18, 18′. The largest diameter on this axiallytapered enlarged portion 31 is preferred to be equal to or smaller thanthe smallest diameter of the mounting portion 18, to aid the axialpassage of the smallest inner diameter of the blade sleeve 22.Preferably also, the radially enlarged portion 31 is axially tapered inthe same manner as the mounting portions 18, 18′. In certainembodiments, a bearing 68 is disposed between the radially enlargedportion 31 and the protection blade sleeve 33. During operation, theprotection blade sleeve 33 will provide a hardened contact point for themill assembly 10 or 50 to assist in casing window exit operations.

Those of skill in the art will recognize that numerous modifications andchanges may be made to the exemplary designs and embodiments describedherein and that the invention is limited only by the claims that followand any equivalents thereof.

What is claimed is:
 1. A mill assembly for cutting in a subterraneanlocation, the mill assembly comprising: a shaft body; and a blade sleevethat radially surrounds a portion of the shaft body, the blade sleevebeing removable from the shaft body.
 2. The mill assembly of claim 1further comprising a bearing disposed radially between the shaft bodyand the blade sleeve.
 3. The mill assembly of claim 2 wherein the shaftbody, blade sleeve and bearing are secured together by press fit so thatthe blade sleeve and bearing are removable from the shaft body.
 4. Themill assembly of claim 2 wherein the shaft body further includes amounting portion upon which the bearing and blade sleeve are mounted,the mounting portion having an outer radial surface that is shaped topreclude rotation of the bearing and blade sleeve with respect to theshaft body.
 5. The mill assembly of claim 2 wherein the bearing isformed of a material that is softer than material forming the bladesleeve and shaft body.
 6. The mill assembly of claim 5 wherein thebearing is formed of at least one of the materials from the groupconsisting of: brass; manganese; bronze; alloys including brass,manganese or bronze; and viscoelastomer.
 7. The mill assembly of claim 4wherein the mounting portion presents an outer radial surface having atleast three arcuately curved contact faces adjoined by corners.
 8. Themill assembly of claim 7 wherein: the mounting portion has a maximumradius as measured from a most distant radial point; and each of thecontact faces presents an outer radial surface having a radius ofcurvature which is greater than the maximum radius of curvature of themounting portion.
 9. The mill assembly of claim 8 wherein, for each ofthe contact faces: a central portion of the contact face has a radius ofcurvature that is greater than the maximum radius of curvature of themounting portion; and a lateral portion of the contact face has a radiusof curvature that is smaller than the maximum radius of curvature of themounting portion.
 10. The mill assembly of claim 4 wherein the mountingportion is axially tapered.
 11. A mill assembly for cutting in asubterranean location, the mill assembly comprising: a shaft body havinga mounting portion; a blade sleeve that radially surrounds the mountingportion of the shaft body; and the mounting portion is axially tapered.12. The mill assembly of claim 11 wherein the shaft body, blade sleeveand bearing are secured together by press fit so that the blade sleeveand bearing are removable from the shaft body.
 13. The mill assembly ofclaim 11 further comprising a bearing that is disposed between the shaftbody and the blade sleeve, the bearing being formed of at least one ofthe materials from the group consisting of: brass; manganese; bronze;alloys including brass, manganese or bronze; and viscoelastomer.
 14. Themill assembly of claim 11 wherein the mounting portion presents an outerradial surface having at least three arcuately curved contact facesadjoined by corners.
 15. The mill assembly of claim 14 wherein there arebetween three and twelve contact faces.
 16. The mill assembly of claim14 wherein: the mounting portion has a maximum radius as measured from amost distant radial point; and each of the contact faces presents anouter radial surface having an axial cross-sectional shape that forms anarcuate segment of a circle having a radius which is greater than themaximum radius of the mounting portion.
 17. A method of assembling amill assembly for cutting in a downhole location, the method comprising:disposing a bearing onto a mounting portion of a mill shaft body;disposing a blade sleeve onto the bearing; and wherein the mill shaftbody, bearing and blade sleeve are press fit together.
 18. The method ofclaim 17 wherein the bearing is formed of a material that is softer thanmaterial forming the blade sleeve and shaft body.
 19. The method ofclaim 17 wherein the mounting portion is axially tapered to facilitatethe press fit.